Conditional (temperature sensitive) mutants affecting flagellar function in Chlamydomonas reinhardi have been obtained. Eight of these have been selected for intensive study of their microtubule proteins. When the temperature is shifted to non-permissive levels, all show immediate defects in flagellar function, some show fragility and loss of flagella, some show altered reaction with cochicine, and one shows additional defects in cell division. It is proposed to complete the biological and genetic characterization of these strains, and the chemical characterization of their flagellar and cellular microtubular proteins. Additional mutants of this type will be obtained and further attempts to obtain colchicine resistant mutants will be made. A second group of mutants which shows defects in flagellar function only after a division cycle at non-permissive temperatures will be used to study flagellar assembly and possible non-chromosomal determinants of basal body function.